Silicon carbide (below, referred to as “SiC”) is excellent in heat resistance and mechanical strength and is stable physically and chemically, so is drawing attention as an environmentally resistant semiconductor material. Further, in recent years, demand has been rising for epitaxial SiC wafers as substrates for high frequency high voltage resistant electronic devices etc.
When using SiC single crystal substrates (below, referred to as “SiC substrates”) to fabricate power devices or high frequency devices etc., usually epitaxial SiC wafers comprised of SiC substrates on which SiC single crystal thin films are epitaxially grown by the thermal CVD process (thermal chemical vapor deposition process) are produced. The reason for further forming an SiC epitaxially grown film on an SiC substrate is to build in devices using layers controlled in doping density. Therefore, if the doping density is insufficiently controlled, the problem is caused that the device characteristics will not be stable. The general practice is to use nitrogen as the SiC doping gas, but nitrogen enters the sites of C in the SiC, so it is known that the smaller the C/Si ratio in the mixed feedstock gases, the easier it is for nitrogen to be taken into the crystal structure. Such an effect is called “site competition”.
If using the thermal CVD process, in general, the method is employed of placing the SiC substrate on a holder in a growth chamber, making the holder rotate while supplying for example feedstock gases comprised of silane gas or chlorosilane gas or other silicon feedstock gas and propane or methane or other hydrocarbon gas mixed together (below, referred to as “mixed feedstock gases”) together with hydrogen or another carrier gas directly over the SiC substrate and thereby epitaxially growing an SiC single crystal thin film (for example, see NPLT 1). At that time, in order to place the SiC substrate on the holder, the general practice is to form the surface of the holder in advance with a recess corresponding to the thickness of the SiC substrate, place the SiC substrate in that recess so that the SiC substrate is carried in a fixed manner, then supply the feedstock gases as explained above from the side of the SiC substrate in a way that blows the feedstock substantially horizontal with respect to the SiC substrate.
When using such a configuration to epitaxially grow an SiC single crystal thin film, in general, the C/Si ratio of the gases differs between the upstream side and downstream side of the flow of gases in the growth chamber. The reason is that if supplying mixed feedstock gases while making the C/Si ratio smaller than 1 so that nitrogen is easily taken into the crystal structure, the carbon and silicon will be consumed in a 1:1 ratio and the gases will deposit as SiC on the SiC substrate, so the relative amount of the carbon in the mixed feedstock gases will fall the further downstream in the flow, and the C/Si ratio will become smaller the further downstream in the flow. Since carbon and silicon are consumed in a 1:1 ratio, if supplying mixed feedstock gases while making the C/Si ratio larger than 1, the C/Si in the mixed feedstock gases will become larger the further downstream in the flow.
In view of this principle, if causing epitaxial growth while making the C/Si ratio in the mixed feedstock gases a ratio of 1, the C/Si ratios at the upstream and downstream sides of the flow of gas inside the growth chamber will not change. However, the C/Si ratio of mixed feedstock gases is an important parameter in the epitaxial growth conditions. To reduce the defect density, reduce bunching, improve in-plane uniformity, and otherwise improve the quality as sought for epitaxial SiC wafers, the pressure, growth temperature, etc. are considered as well and in general a ratio of other than 1 is selected.
For example, PLT 1 discloses a process for production of a silicon carbide single crystal wafer including a step of making a silicon source gas and a carbon source gas react to epitaxially grow an α-type silicon carbide single crystal on a wafer. PLT 1 discloses that the ratio of supply (C/Si) of carbon (C) in the carbon source gas and silicon (Si) in the silicon source gas is preferably 0.5 to 1.4 from the viewpoint of excellent epitaxial growth and the viewpoint of prevention of macrotriangular pit defects.
To deal with the above such problem of the C/Si ratio not becoming uniform, it is in principle possible to make the holder of the epitaxial system using the thermal CVD process itself rotate so as to cancel out changes in the environment between the upstream side and downstream side of the flow of gas. However, in actuality, it is broadly known that if performing epitaxial growth, the doping density at the holder in the planar direction is not sufficiently leveled (for example, see NPLT 2). This suggests that a different situation is occurring between upstream and downstream of the flow of gas other than changes in the C/Si ratio in the mixed feedstock gases.